A. de Siervo

Fe and Mo Valences in Sr2FeMoO6

Abstract The valences of Fe and Mo in Sr 2 FeMoO 6 are investigated by means of X-ray absorption and photoemission spectroscopies. We find that the d occupations are similar to those of Fe in Fe 2 O 3 and Mo in MoO 3 . Considering the covalency, however, we argue that the ground state is closer to a Fe +3 (3 d 5 )–Mo +5 (4 d 1 ) configuration than to a Fe +2 (3 d 6 )–Mo +6 (4 d 0 ) configuration.

Hafnium silicide formation on Si(100) upon annealing

High dielectric constant materials, such as HfO{sub 2}, have been extensively studied as alternatives to SiO{sub 2} in new generations of Si based devices. Hf silicate/silicide formation has been reported in almost all literature studies of Hf based oxides on Si, using different methods of preparation. A silicate interface resembles close to the traditional Si/SiO{sub 2}. The silicate very likely forms a very sharp interface between the Si substrate and the metal oxide, and would be suitable for device applications. However, the thermal instability of the interfacial silicate/oxide film leads to silicidation, causing a dramatic loss of the gate oxide integrity. Despite the importance of the Hf silicide surface and interface with Si, only a few studies of this surface are present in the literature, and a structural determination of the surface has not been reported. This paper reports a study of the Hf silicide formation upon annealing by using a combination of XPS, LEED, and x-ray photoelectron diffraction (XPD) analyses. Our results clearly indicate the formation of a unique ordered Hf silicide phase (HfSi{sub 2}), which starts to crystallize when the annealing temperature is higher than 550 deg. C.

The U11 PGM beam line at the Brazilian National Synchrotron Light Laboratory

We present the U11 beam line at the National Synchrotron Light Laboratory (LNLS), Brazil. This is the first undulator based beam line installed at the UVX storage ring. It is based on the collimated plane grating monochromator design, which allows for variation in included angle of the grating. The results obtained during the commissioning demonstrate a good overall performance in terms of energy resolution and beam size, in accord with the expected values obtained from ray tracing calculation.

Electronic structure of Sr2FeMoO6

Abstract We have used electron spectroscopies to investigate the electronic structure of the double perovskite Sr2FeMoO6. The valence-band photoemission spectra present a well-defined cut-off at the Fermi level, indicative of the metallic character of the material. The O 1s X-ray absorption spectrum presents three peaks, which are in good correspondence with the main structures in the unoccupied density-of-states of DF-LDA calculations. The electron energy-loss spectra present two structures which are also interpreted in terms of transitions between the bands obtained in the DF-LDA calculations.

Measurement of the 2p XPS spectra of 4d metals: Nb to Sb

Abstract Experimental determinations of XPS core-level spectra have been essential in elucidating the physics of excitation and recombination mechanisms in trasition and noble metals. In the 3d metal series, comparison of spectra of levels in the L and M X-ray shells has, for example, reinforced confidence in theories explaining asymmetries and satellites. In the 4d metals, measurements of M and N core-level spetra support similar theoretical interpretations. Measurements of L spectra have not been reported, however, because the deep 2s and 2p levels are not accessible to conventional sources. We present the results of high-resolution measurements of these levels in Nb, Mo, Ru, Rh, Pd, Ag, In, Sn and Sb. The spectra were excited with a Ti anode and an efficient separation algorithm permitted isolation of the TiKα1 andKα2 contributions: results for the 3p spectra were consistent with those measured with an Al anode. The2p3/2 spectra were measurable in all the metals, as were the2p1/2 in the metals lighter than Ag. The 2s were measurable in Ru, Rh, Pd and Ag. As a by-product of these measurements, we report results for satellite spectra and loss structures. The line widths were also determined and we compare them with results from atomic theory and from indirect measurements utilizing X-rays, indicating a satisfactory agreement.

A new ultra-high-vacuum variable temperature and high-magnetic-field X-ray magnetic circular dichroism facility at LNLS.

X-ray magnetic circular dichroism (XMCD) is one of the most powerful tools for investigating the magnetic properties of different types of materials that display ferromagnetic behavior. Compared with other magnetic-sensitive techniques, XMCD has the advantage of being element specific and is capable of separating the spin and magnetic moment contributions associated with each element in the sample. In samples involving, for example, buried atoms, clusters on surfaces or at interfaces, ultrathin films, nanoparticles and nanostructures, three experimental conditions must be present to perform state-of-the-art XMCD measurements: high magnetic fields, low temperatures and an ultra-high-vacuum environment. This paper describes a new apparatus that can be easily installed at different X-ray and UV beamlines at the Brazilian Synchrotron Light Laboratory (LNLS). The apparatus combines the three characteristics described above and different methods to measure the absorption signal. It also permits in situ sample preparation and transfer to another chamber for measurement by conventional surface science techniques such as low-energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED), X-ray photoelectron spectroscopy (XPS) and X-ray photoelectron diffraction (XPD). Examples are given of XMCD measurements performed with this set-up on different materials.

Photoelectron diffraction study of Rh nanoparticles growth on Fe3O4/Pd(111) ultrathin film

Metallic nanoparticles (NPs) supported on oxides thin films are commonly used as model catalysts for studies of heterogeneous catalysis. Several 4d and 5d metal NPs (for example, Pd, Pt and Au) grown on alumina, ceria and titania have shown strong metal support interaction (SMSI), for instance the encapsulation of the NPs by the oxide. The SMSI plays an important role in catalysis and is very dependent on the support oxide used. The present work investigates the growth mechanism and atomic structure of Rh NPs supported on epitaxial magnetite Fe3O4(111) ultrathin films prepared on Pd(111) using the Molecular Beam Epitaxy (MBE) technique. The iron oxide and the Rh NPs were characterized using X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction and photoelectron diffraction (PED). The combined XPS and PED results indicate that Rh NPs are metallic, cover approximately 20 % of the iron oxide surface and show height distribution ranging 3–5 ML (monolayers) with essentially a bulk fcc structure.

Solving the thermal stability problem at the HfO2/Si interface with previous N implantation

We report on the use of N implantation to produce thin, pure, uniform, and thermally stable Si3N4 buffer layers and HfO2 overlayers on Si(100) and Si(111) without silicide formation.We report on the use of N implantation to produce thin, pure, uniform, and thermally stable Si3N4 buffer layers and HfO2 overlayers on Si(100) and Si(111) without silicide formation.

LINE SHAPE VARIATIONS OF AG AUGER SHAKE-UP SATELLITES OUTSIDE THE RESONANCE REGIME

Abstract In recent publications, unambiguously identified shake-up satellites have been observed in the L123M45M45 spectra of 4d metals, studied with conventional X-ray sources and, in the L3 threshold region, with synchrotron radiation. There remain questions concerning the origin of these satellites and their relation to the corresponding X-ray photoelectron spectroscopy (XPS) spectra. We report here a study of the Ag L23M45M45 spectra as a function of photon energy and demonstrate the evolution of the line shapes of the satellite spectra. The transition from the adiabatic to sudden approximation regimes appears to occur at around 80 eV above threshold, which is much less than expected for plasmon satellites. These results may reflect the influence of quasi-atomic screening, as opposed to metallic screening. The experiments were performed using the SXS-beamline at the National Synchrotron Laboratory of Brazil (LNLS).

Formation of Rh islands on Pd-supported α-Fe2O3 (0001)

A rhodium ultra-thin film was deposited on a Pd-supported α-Fe2O3 (0001) film by molecular beam epitaxy (MBE). The surface atomic and electronic structures were studied using X-ray photoelectron diffraction (XPD), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED). For an oxide thin film (∼16.5 A), an ordered (√3 × √3)R30° structure was observed. The multiple scattering calculation approach combined with a genetic algorithm for surface structure optimization was used to investigate precisely the atomic structure of the oxide support as well as the Rh nanoislands or nanoparticles (NPs). Rh exhibited 3D growth on an α-Fe2O3 thin film forming NPs with a lattice constant expanded by 5.26% relative to the Rh bulk value. The coexistence of 60° rotated domains and also evidence of Fe termination at the surface of α-Fe2O3 were observed.